Dynamic mechanical behaviors of pearlitic U71MnG rail steel: Deformation mechanisms and constitutive model

Pearlitic rail steels with excellent mechanical and wear properties are widely employed for high-speed railways. During the service period, the presence of track irregularities, switches, as well as crossings can cause intense dynamic loadings, which accumulate the severe deformation of rails, and sometimes even facilitate crack initiation and failure. Knowledge of the dynamic mechanical response and damage behaviors of the rail steels is therefore important for the service performance estimation of railways. In this study, the pearlite U71MnG rail steel was deformed using a split Hopkinson pressure bar (SHPB) apparatus to reach a wide range of strain rates and temperatures. Microstructures including the grain morphology, deformation characteristic of cementite lamellae, dislocation substructures in ferrite lamellae, and phase transformation at pearlite colony boundaries were characterized. Based on the microstructural observations, the rate-controlling strengthening mechanism and temperature-dependent softening mechanism of the steel were revealed when the process for the dynamic loading-induced cementite decomposition at boundaries was discussed. In consideration of the athermal deformation mechanism, thermally activated mechanism, and viscous-drag effect, a physically based constitutive model was proposed to describe the dynamic mechanical response of the pearlite rail steel. The predicted flow stresses were found to be consistent with the experimental one for all strain rates and temperatures.